Muon data confirms fuel melt at Fukushima Daiichi 1

23 March 2015

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Initial results from using a muon detection system at the damaged Fukushima Daiichi unit 1 in Japan appear to confirm that most of the fuel has melted and dropped from its original position within the core, Tokyo Electric Power Company (Tepco) announced.

Results obtained from the muon detector on the northwest side of the reactor building (Image: Tepco)

The company completed installation of the muon detection system on 12 February. Two detectors were installed: one on the northwest side of the reactor building and the other on the north side. Since then, data collection continued until 10 March (a period of 26 days). The initial results have now been analysed.

The detector system was developed by Japan's High Energy Accelerator Research Organization (KEK). The system uses the so-called permeation method to measure the muon data.

Muons are high-energy subatomic particles that are created when cosmic rays enter Earth's upper atmosphere. These particles naturally and harmlessly strike the Earth's surface at a rate of some 10,000 muons per square meter per minute. Muon tracking devices detect and track these particles as they pass through objects. Subtle changes in the trajectory of the muons as they penetrate materials and change in direction correlate with material density. Nuclear materials such as uranium and plutonium are very dense and are therefore relatively easy to identify.

Tepco said that, while the images produced using the initial data were not clear, "some structure and equipment edges were distinguished". It says they show the primary containment vessel, the reactor pressure vessel and the core "are in positions where they should be". However, Tepco said the measurement data "do not show the existence of high-density substances (fuels) in the original position of the reactor core".

The results also show "a strong possibility that there is no water accumulated in the core area of the reactor pressure vessel," Tepco said.

"This result is basically consistent with Tepco's previously announced estimation of the reactor and the containment vessel conditions," the company said. "The result measured this time will become very relevant information in order to grasp the debris location to proceed with the decommissioning operation."

Tepco plans to conduct an investigation of the lower part of the unit's containment vessel using robots in order to "identify the distribution status of fuel debris remaining in the lower part of the reactor".